I thought at first that I knew where this issue could come from: Docker by default runs in HTTP mode and the crypto libraries we use for client side encryption stop working if the connection to the server is not encrypted. REI3 of course should run in any event - but since almost no-one (to my knowledge) uses REI3 in HTTP mode (besides for testing/development), a bug like this could have gone unnoticed.
But I´ve started a test instance in HTTP mode and was also not able to replicate the issue.
Could you please provide a screenshot, on what you see, when you get the error? Bonus points if you can also show the browser developer tools console (usually opened via F12). Maybe if I can see the specific error, I can say more.